I have about 400 Books and a multitude of tags. I usually download "social metadata" and keep the tags. I just add my own tags.

Would it be possible to create collections only by certain tags from a list? (e.g. Non-fiction, Romance, Classic literature, Fantasy, …) and leave all other tags aside?

imo tags are a lot easier to edit than custom columns, but i do tag about contents that i would not create my collections on.

I've been thinking about your issue but I can't think of an easy way to do what you would like to do. In my case, I have 66 total unique tags, but only 48 that I wanted collections from. Rather than editing the metadata for the individual books, I appended "-nocollect" (could be -nc or even -n) to the tags I didn't want collections for (from the tag browser - right-click>rename) and added that as an ends-with pattern in the ignore list. This worked for me, but if you have a significant number of "unwanted" tags it could be tedious.

I know you prefer not to use custom columns, but if you did add a custom "genre" column, you could then filter your list to the books already containing "Romance" tag, for example, and edit the custom metadata for all of those in one go. Still tedious, but at least you'd be working with groups of books rather than individual books.

My suggestions may be no help at all, and you may have already considered all of this, but I just thought I'd throw it out there.